The use of solid dry lubricant particles in grinding wheels is well-known; the use of graphite particles in a vitrified or glass bonded wheel is taught in U.S. Pat. No. 3,454,384, to Kumagai, and in U.S. Pat. No. 4,157,897 to Keat, the inventor herein.
The use of particulate graphite in grinding wheels provides a method of wheel grade control (hardness of the wheel), improves the thermal conductivity (thus lowering the temperature at the grinding face), acts as a lubricant (lowering the generation of heat), and acts to prevent formation of a coherent film of workpiece material or bond material which would interfere with grinding and cause loading of the wheel.
One disadvantage of using graphite in glass bonded wheels is the fact that known glassy bonds do not readily wet the graphite particles, and thus it is difficult to achieve a low porosity wheel.
The Kumagai patent relates to wheels which have a bond having the constitution of a hard graphite pencil and thus include a large amount of graphite in a fired clay matrix. The bond includes a large amount of porosity which is impregnated with a lubricating agent such as stearic acid which melts at or below the grinding temperature.
The Keat patent relates to hot pressed wheels and includes no temporary "green" binder in the mix.
U.S. Pat. No. 3,402,035 to Martin teaches the use of metal clad graphite in a resin or metal bonded diamond wheel to improve the bonding of the graphite to the metal or resin matrix.
Conventional bonds for glass (vitrified) bonded grinding wheels contain organic temporary or "green" bonding material such as starches or sugars to hold the wheel together before the glass bond is developed by firing. During the manufacture of the wheel, or other grinding tool, after molding and before firing to the vitrification temperature, it is necessary to remove the organic binding by heating the wheels in an oxidizing atmosphere. Such oxidizing conditions, adequate to remove the temporary binder, also would remove any graphite or other equally readily oxidizable conventional dry film lubricant. Thus conventional manufacturing techniques for making ceramic or glass bonded grinding tools (vitrified bonded) do not permit the use of conventional dry film lubricants such as graphite, molybdenum sulfide, hexagonal boron nitride, and zinc sulfide.